Aluminum sheet products having improved fatigue crack growth resistance and methods of making same

ABSTRACT

Aluminum sheet products having highly anisotropic grain microstructures and highly textured crystallographic microstructures are disclosed. The products exhibit improved strength and improved resistance to fatigue crack growth, as well as other advantageous properties such as improved combinations of strength and fracture toughness. The sheet products are useful for aerospace and other applications, particularly aircraft fuselages.

FIELD OF THE INVENTION

The present invention relates to the production of rolled aluminumproducts having improved properties. More particularly, the inventionrelates to the manufacture of aluminum sheet products having controlledmicrostructures, which exhibit improved strength and fatigue crackgrowth resistance. The sheet products are useful for aerospaceapplications such as aircraft fuselages, as well as other applications.

BACKGROUND INFORMATION

Aircraft components such as fuselages are typically fabricated fromaluminum sheet products. Resistance to the growth of fatigue cracks insuch aerospace products is very important. Better fatigue crack growthresistance means that cracks will grow slower, thus making aircraftsafer because small cracks can be more readily detected before theyachieve a critical size which could lead to a catastrophic failure. Inaddition, slow crack growth can have an economic benefit because longerinspection intervals may be used. U.S. Pat. No. 5,213,639 to Colvin etal. discloses aluminum alloy products useful for aircraft applications.

The present invention provides rolled aluminum sheet products havingimproved resistance to fatigue crack growth, as well as otheradvantageous properties including improved combinations of strength andfracture toughness.

SUMMARY OF THE INVENTION

Aluminum sheet products fabricated in accordance with the presentinvention exhibit improved resistance to the propagation of cracks.Aluminum alloy compositions and processing parameters are controlled inorder to increase fatigue crack growth resistance. This resistance is aresult of a highly anisotropic grain microstructure which forces cracksto experience a transgranular or an intergranular tortuous propagationpath. The number of cycles required to propagate these tortuous cracksto a critical crack length is significantly greater than the number ofcycles required to propagate a crack that follows a smooth intergranularor non-tortuous path.

In an embodiment of the invention, alloy compositions, thermo-mechanicaland thermal practices are controlled in order to develop anunrecrystallized microstructure or a desired amount ofrecrystallization. The microstructures are controlled with the help ofdispersoids or precipitates which are formed at intermediate processingsteps, or precipitation treatments to yield obstacles for dislocationand grain boundary motion. The sheet products comprise elongated grains,which form a highly anisotropic microstructure.

In accordance with one embodiment, the anisotropic microstructure may bedeveloped as a result of hot rolling and additional thermal practices.The hot rolling temperature is controlled in order to facilitate thedesired type, volume fraction and distribution of crystallographictexture. In one embodiment, a recovery anneal after hot rolling yieldsthe desired anisotropic microstructure after final solution heattreating and optional stretching and tempering operations. Additionalintermediate anneals may be used to control the driving force forrecrystallization.

The compositions of the aluminum products are preferably selected inorder to provide dispersoid forming alloying elements, which controlrecrystallization and recovery processes during production. In oneembodiment, mixtures of alloying elements that form the coherent Cu₃Auprototype structure (L12 in the structurebereight nomenclature) arepreferred. Such elements include Zr, Hf and Sc. In addition, alloyingelements that form incoherent dispersoids such as Cr, V, Mn, Ni and Femay also be utilized. Combinations of such alloying elements may beused.

An aspect of the present invention is to provide a rolled aluminum alloysheet product having high levels of crystallographic anisotropy.

Another aspect of the present invention is to provide an Al—Cu basealloy sheet product having high levels of crystallographic anisotropy.

A further aspect of the present invention is to provide an aircraftfuselage sheet comprising a rolled aluminum alloy sheet product havingan anisotropic microstructure.

Another aspect of the present invention is to provide a method of makingan aluminum alloy sheet product having a highly anisotropic grainmicrostructure. The method includes the steps of providing an aluminumalloy, hot rolling the aluminum alloy to form a sheet,recovery/recrystallize annealing the hot rolled sheet, solution heattreating the annealed sheet, and recovering a sheet product having ananisotropic microstructure.

These and other aspects of the present invention will be more apparentfrom the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially schematic drawing of an airplane including analuminum alloy fuselage sheet, indicating the orientation of typicalfatigue cracks which tend to develop in the fuselage sheet.

FIG. 2 is a fabrication map for an aluminum sheet product having ananisotropic microstructure produced in accordance with an embodiment ofthe present invention.

FIG. 3 is a fabrication map for an aluminum sheet product having ananisotropic microstructure produced in accordance with anotherembodiment of the present invention.

FIGS. 4 a and 4 b are photomicrographs illustrating the substantially“equiaxed” grains of Aluminum Association alloy 2024 and 2524 sheetproducts which are conventionally used as fuselage sheet.

FIGS. 5 a and 5 b are photomicrographs illustrating the anisotropicmicrostructure of an aluminum sheet product produced in accordance withan embodiment of the present invention.

FIGS. 6 a and 6 b are photomicrographs illustrating the anisotropicmicrostructure of another aluminum sheet product produced in accordancewith an embodiment of the present invention.

FIGS. 7 a and 7 b are photomicrographs illustrating the anisotropicmicrostructure of a further aluminum sheet product produced inaccordance with an embodiment of the present invention.

FIGS. 8 a and 8 b are photomicrographs illustrating the anisotropicmicrostructure of another aluminum sheet product produced in accordancewith an embodiment of the present invention.

FIGS. 9 a and 9 b are photomicrographs illustrating the anisotropicmicrostructure of a further aluminum sheet product produced inaccordance with an embodiment of the present invention.

FIGS. 10 a and 10 b are photomicrographs illustrating the anisotropicmicrostructure of another aluminum sheet product produced in accordancewith an embodiment of the present invention.

FIG. 11 illustrates the layout of specimens taken from sheet samples fortesting.

FIG. 12 is a graph illustrating tensile yield strength values for sheetsamples of the present invention in different orientations.

FIGS. 13 and 14 are graphs illustrating crack growth resistance curvesfor sheet samples of the present invention.

FIG. 15 is a graph illustrating fracture toughness and tensile yieldstrength for sheet samples of the present invention.

FIG. 16 is a graph illustrating fatigue test results for two of thepresent alloys exhibiting unrecrystallized microstructures.

FIG. 17 is a graph illustrating tensile yield strengths for sheetsamples of the present invention in different orientations.

FIG. 18 is a photomicrograph illustrating the anisotropic microstructureof an aluminum sheet product produced in accordance with an embodimentof the present invention.

FIG. 19 is a photomicrograph illustrating the anisotropic microstructureof another aluminum sheet product produced in accordance with anembodiment of the present invention.

FIG. 20 is a photomicrograph illustrating the anisotropic microstructureof a farther aluminum sheet product used in accordance with anembodiment of the present invention.

FIG. 21 is a photomicrograph illustrating the anisotropic microstructureof another aluminum sheet product produced in accordance with anembodiment of the present invention.

FIG. 22 is a graph illustrating tensile yield strength values for sheetproducts of the present invention in different orientations.

FIGS. 23-26 are graphs illustrating fracture toughness and tensile yieldstrength values for sheet products produced in accordance withembodiments of the present invention.

FIG. 27 is a graph illustrating duplicate fatigue test results for twoalclad alloys exhibiting elongated recrystallized grains.

FIG. 28 is a graph illustrating results from S/N fatigue testing for twoalclad alloys exhibiting elongated recrystallized grains.

DETAILED DESCRIPTION

In accordance with the present invention, a rolled aluminum alloy sheetproduct is provided which comprises a highly anisotropic microstructure.As used herein, the term “anisotropic microstructure” means a grainmicrostructure where the grains are elongated unrecrystallized grains orelongated recrystallized grains with an average aspect ratio of lengthto thickness of greater than about 4 to 1. The average grain aspectratio is preferably greater than about 6 to 1, more preferably greaterthan about 8 to 1. In a particularly preferred embodiment, theanisotropic microstructure has an average grain aspect ratio of greaterthan about 10 to 1. In both instances of recrystallized orunrecrystallized grains, the common feature among recrystallized andunrecrystallized grain microstructures is that the grains are elongated.Observation of these grains may be done, for example, by opticalmicroscopy at 50× to 100× in properly polished and etched samplesobserved through the thickness in the longitudinal orientation. Forrecrystallized products, the anisotropic microstructures achieved inaccordance to the present invention preferably exhibit a Goss texture,as determined by standard methods, of greater than 20, more preferablygreater than 30 or 40. For unrecrystallized products, the anisotropicmicrostructures preferably exhibit a Brass texture, as determined bystandard methods, of greater than 20, more preferably greater than 30 or40.

As used herein, the term “sheet” includes rolled aluminum productshaving thicknesses of from about 0.01 to about 0.35 inch. The thicknessof the sheet is preferably from about 0.025 to about 0.325 inch, morepreferably from about 0.05 to about 0.3 inch. For many applications suchas some aircraft fuselages, the sheet is preferably from about 0.05 toabout 0.25 inch thick, more preferably from about 0.05 to about 0.2inch. The sheet may be unclad or clad, with preferred cladding layerthicknesses of from about 1 to about 5 percent of the thickness of thesheet.

As used herein, the term “unrecrystallized” means a sheet product thatexhibits grains that relate to the original grains present in the ingotor intermediate slab. The original grains have only been physicallydeformed. As a result, the unrecrystallized grain microstructures alsoexhibit a strong hot rolling crystallographic texture. The term“recrystallized” as used herein means grains that have formed from theoriginal deformed grains. This occurs typically during hot rolling,during solution heat treating or during anneals, these anneals can beintermediate between hot rolling and/or prior to solution heat treating.

In one embodiment of the invention, the sheet products are useful asaircraft fuselage sheet. FIG. 1 schematically illustrates an airplane 10including a fuselage 12 which may be made of the present wroughtaluminum alloy sheet. The aluminum alloy sheet may be provided with atleast one aluminum cladding layer by methods known in the art. The clador unclad sheet of the present invention may be assembled as an aircraftfuselage in a conventional manner known in the art. The arrows A and Bin FIG. 1 indicate the orientations and propagation paths of fatiguecracks, which tend to develop in airplane fuselage sheet. In accordancewith an embodiment, the anisotropic microstructure of the present sheetproduct is oriented on the fuselage such that the lengths of the highaspect ratio grains are substantially perpendicular to the likelyfatigue crack propagation paths through the fuselage sheet. For example,either the longitudinal and/or long transverse orientations of the sheetmay be positioned substantially perpendicular to the directions A or Bshown in FIG. 1.

In accordance with the present invention, aluminum alloy compositionsare controlled in order to increase fatigue crack growth resistance.Some suitable alloy compositions may include Aluminum Association 2xxx,5xxx, 6xxx and 7xxx alloys, and variants thereof. For example, suitablealuminum alloy compositions for use in accordance with the presentinvention include Al—Cu base alloys, such as 2xxx alloys. A preferredAl—Cu base alloy comprises from about 1 to about 5 weight percent Cu,more preferably at least about 3 weight percent Cu, and from about 0.1to about 6 weight percent Mg.

An example of a particularly preferred Al—Cu base alloy comprises fromabout 3.5 to about 4.5 weight percent Cu, from about 0.6 to about 1.6weight percent Mg, from about 0.3 to about 0.7 weight percent Mn, andfrom about 0.08 to about 0.13 weight percent Zr. In accordance withanother preferred embodiment, the rolled aluminum alloy sheet producthas a composition of from about 3.8 to about 4.4 weight percent Cu, fromabout 0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6weight percent Mg, and from about 0.09 to about 0.12 weight percent Zr.In accordance with a further preferred embodiment, the rolled aluminumsheet product has a composition of from about 3.4 to about 4.0 weightpercent Cu, from 0 to about 0.4 weight percent Mn, from about 1.0 toabout 1.6 weight percent Mg, and from about 0.09 to about 0.12 weightpercent Zr. In accordance with another preferred embodiment, the rolledaluminum alloy sheet product has a composition of from about 3.2 toabout 3.8 weight percent Cu, from about 0.3 to about 0.7 weight percentMn, from about 1.0 to about 1.6 weight percent Mg, from about 0.09 toabout 0.12 weight percent Zr and from about 0.25 to about 0.75 weightpercent Li.

The Al—Cu base alloys produced in accordance with the present inventionmay comprise up to about 1 weight percent of at least one additionalalloying element selected from Zn, Ag, Li and Si. These elements, whenproperly heat treated, may give rise to the formation of strengtheningprecipitates. Such precipitates form during natural aging at roomtemperature or during artificial aging, e.g., up to temperatures of 350°F.

The Al—Cu base alloys may further comprise up to about 1 weight percentof at least one additional alloying element selected from Hf, Sc, Zr andLi. These elements, when properly heat treated, may give rise to theformation or enhancement of coherent dispersoids. Such dispersoids mayenhance the ability of the microstructure to be produced with elongatedrecrystallized or unrecrystallized grains.

The Al—Cu base alloys may further comprise up to about 1 weight percentof at least one additional alloying element selected from Cr, V, Mn, Niand Fe. These elements, when properly heat treated, may give rise to theformation of incoherent dispersoids. Such dispersoids may help tocontrol recrystallization and grain growth.

In addition to Al—Cu base alloys, Al—Mg base alloys, Al—Si base alloys,Al—Mg—Si base alloys and Al—Zn base alloys may be produced as sheetproducts having anisotropic microstructures in accordance with thepresent invention. For example, Aluminum Association 5xxx, 6xxx and 7xxxalloys, or modifications thereof, may be fabricated into sheet productshaving anisotropic microstructures.

Suitable Al—Mg base alloys have compositions of from about 0.2 to about7.0 weight percent Mg, from 0 to about 1 weight percent Mn, from 0 toabout 1.5 weight percent Cu, from 0 to about 3 weight percent Zn, andfrom 0 to about 0.5 weight percent Si. In addition, Al—Mg base alloysmay optionally include further alloying additions of up to about 1weight percent strengthening additions selected from Li, Ag, Cd andlanthanides, and/or up to about 1 weight percent dispersoid formers suchas Cr, Fe, Ni, Sc, Hf, Ti, V and Zr.

Suitable Al—Mg—Si base alloys have compositions of from about 0.1 toabout 2.5 weight percent Mg, from about 0.1 to about 2.5 weight percentSi, from 0 to about 2 weight percent Cu, from 0 to about 3 weightpercent Zn, and from 0 to about 1 weight percent Li. In addition,Al—Mg—Si base alloys may optionally include further alloying additionsof up to about 1 weight percent strengthening additions selected fromAg, Cd and lanthanides, and/or up to about 1 weight percent dispersoidformers such as Mn, Cr, Ni, Fe, Sc, Hf, Ti, V and Zr.

Suitable Al—Zn base alloys have compositions of from about 1 to about 10weight percent Zn, from about 0.1 to about 3 weight percent Cu, fromabout 0.1 to about 3 weight percent Mg, from 0 to about 2 weight percentLi, and from 0 to about 2 weight percent Ag. In addition, Al—Zn basealloys may optionally include further alloying additions of up to about1 weight percent strengthening additions selected from Cd andlanthanides, and/or up to about 1 weight percent dispersoid formers suchas Mn, Cr, Ni, Fe, Sc, Hf, Ti, V and Zr.

In accordance with the present invention, processing parameters arecontrolled in order to increase fatigue crack growth resistance of therolled aluminum alloy sheet products. A preferred process includes thesteps of casting, scalping, preheating, initial hot rolling, reheating,finish hot rolling, optional cold rolling, optional intermediate annealsduring hot rolling and/or cold rolling, annealing for the control ofanisotropic grain microstructures, solution heat treating, flatteningand stretching and/or cold rolling. An example of a fabrication map isshown in FIG. 2. Another example of a fabrication may is shown in FIG.3.

As illustrated in FIG. 2, a recovery anneal step is preferably utilizedin the production of sheet products in accordance with the presentinvention. As illustrated in FIG. 3, intermediate anneals during hotrolling and/or cold rolling may be used in addition to, or in place of,the recovery anneal. It should be noted that the anneals can be providedby controlled heating or by single or multiple holding times at one orseveral temperatures.

Depending on the particular alloy composition, the preheating step ispreferably carried out at a temperature of between 800 and 1,050° F. for2 to 50 hours. The initial hot rolling is preferably performed at atemperature of from 750 to 1,020° F. with a reduction in thickness offrom 0.1 to 3 inch percent per pass. Reheating is preferably carried outat a temperature of from 700 to 1,050° F. for 2 to 40 hours. The finishhot rolling step is preferably performed at a temperature of from 680 to1,050° F. with a reduction in thickness of from 0.1 to 3 inch per pass.

The optional intermediate anneals during hot rolling or cold rolling,e.g., as illustrated in FIG. 3, are preferably carried out at atemperature of between about 400 and about 1,000° F. for 0.5 to 24hours.

The cold rolling step is preferably carried out at room temperature witha reduction in thickness of from 5 percent to 50 percent per pass.

The recovery/elongated grain recrystallization anneals, e.g., asillustrated in FIG. 2, are preferably carried out at a temperature ofbetween about 300 and about 1,000° F. for 0.5 to 96 hours.Unrecrystallized anisotropic microstructures typically require annealsat relatively low temperatures, for example, from about 400 to about700° F. Recrystallized anisotropic microstructures typically requireanneals at relatively high temperatures, for example, from about 600 toabout 1,000° F.

Solution heat treatment is preferably carried out at a temperature offrom about 850 to about 1,060° F. for a time of from about 1 to 2minutes to about 1 hour.

The quenching step is preferably carried out by rapid cooling usingimmersion into a suitable cooling fluid or by spraying a suitablecooling fluid.

The flattening and stretching steps are preferably carried out toprovide no more than 6 percent of total cold deformation.

After solution heat treatment, cold working may optionally be performed,preferably by stretching or cold rolling. The cold working processpreferably imparts a maximum of 15 percent cold deformation to the sheetproduct, more preferably a maximum of about 8 percent.

The sheet products fabricated in accordance with the present inventionexhibit substantially increased strength and/or resistance to the growthof fatigue cracks as a result of their anisotropic microstructures. In apreferred embodiment, the rolled sheet products exhibit longitudinal (L)tensile yield strengths (TYS) greater than 45 ksi, more preferablygreater than 48 ksi. The rolled sheet products preferably exhibit longtransverse (LT) tensile yield strengths greater than 40 ksi, morepreferably greater than 43 ksi. In the long transverse (T-L)orientation, the rolled sheet in the T3 temper preferably exhibits afatigue crack growth rate da/dN of less than about 5×10⁻⁶ inch/cycle ata ΔK of 10 ksi√inch, more preferably less than about 4×10⁻⁶ or 3×10⁻⁶inch/cycle. In the T36 temper, the rolled sheet exhibits a T-Lorientation fatigue crack growth rate da/dN of less than about 4×10⁻⁶inch/cycle at a ΔK of 10 ksi√inch, more preferably less than 3×10⁻⁶ or2×10⁻⁶ inch/cycle.

Furthermore, the present wrought aluminum alloy sheet products exhibitimproved fracture toughness values, e.g., as tested with 16 by 44 inchcenter notch fracture toughness specimens in accordance with ASTM E561and B646 standards. For example, sheet products produced in accordancewith the present invention preferably exhibit longitudinal (L-T) or longtransverse (T-L) K_(c) fracture toughness values of greater than 130 or140 ksi√inch. The sheet products also preferably possess L-T or T-LK_(app) fracture toughness values of greater than 85 or 90 ksi√inch.

Thus, in addition to improved fatigue crack growth resistance, thepresent sheet products exhibit improved combinations of strength andfracture toughness.

FIGS. 4 a and 4 b are photomicrographs illustrating the substantiallyequiaxed grains of conventional alloy 2024 and 2524 sheet products whichare used as fuselage sheet. Unlike conventional fuselage sheet such asshown in FIGS. 4 a and 4 b, the anisotropic microstructure of thepresent sheet products enables aircraft manufacturers to orient thesheet in directions which take advantage of the increased mechanicalproperties of the sheet, such as improved longitudinal and/or longtransverse fatigue crack growth resistance, fracture toughness and/orstrength.

Table 1 below lists compositions of some sheet products, which may beprocessed to provide anisotropic microstructures in accordance withembodiments of the present invention. TABLE 1 Sheet Product AlloyCompositions (Weight Percent) Alloy Sample No. Cu Mn Mg Zr Sc Li Fe SiAl 770-308 (Zr alloy) 3.74 0 1.36 0.12 0 0 0.03 0.04 balance 770-311(Zr + Li alloy) 3.19 0 1.22 0.10 0 0.31 0.03 0.04 balance 770-309 (Mn +Zr alloy) 4.26 0.57 1.4 0.10 0 0 0.07 0.04 balance 770-310 (Zr + Scalloy) 3.7 0 1.36 0.10 0.06 0 0.04 0.03 balance 770-312 (Zr + Sc + Lialloy) 3.56 0 1.36 0.10 0.06 0.31 0.04 0.03 balance 596-367 (Mn + Zr +Li alloy) 3.37 0.58 1.21 0.12 0 0.76 0.04 0.02 balance

The sheet products having compositions listed in Table 1 were made asfollows. Ingots measuring 6 inches×16 inches×60 inches were cast usingdirect chill (DC) molds. The compositions reported in Table 1 weremeasured from metal samples obtained from the molten metal bath. Ingotswere first stress relieved by heating to 750° F. for 6 hours. The ingotswere then scalped to remove 0.25 inch surface layer from both rollingsurfaces and side sawed to 14 inch width. For preheating, ingots wereheated to 850° F., soaked for 2 hours, then heated to 875° F. and soakedan additional 2 hours. Ingots taken from the preheating furnace werecross rolled 22 percent to a 4.5 inch gauge followed by lengthening to a2 inch gauge. Metal temperature was maintained above 750° F. withreheats to 850° F. for 15 minutes. The 2 inch slab was sheared in halfand reheated to 915° F. for 8 hours, table cooled to 900° F. and hotrolled to 0.25 inch gauge. Suitable reheats were provided during hotrolling to 915° F. for 15 minutes. Metal temperature was kept above 750°F. After hot rolling, sheet product 0.150 inch gauge was fabricated.Recovery anneals prior to solution heat treatment of from 8 to 24 hoursat temperatures from 400° F. to 550° F. yielded unrecrystallizedmicrostructures after solution heat treatment.

After rolling, solution heat treating and quenching, all pieces of sheetwere ultrasonically inspected to Class B and they all passed.Microstructural analyses revealed that all samples exhibitedunrecrystallized microstructures in the final temper. FIGS. 5 a to 10 bare photomicrographs illustrating the anisotropic microstructures of thesheet products listed in Table 1. In each case, the sheet possesses highlevels of crystallographic anisotropy and exhibits elongated grains. Thegrain anisotropy is most pronounced in the longitudinal direction (L) ofeach sheet, but is also present in the long transverse direction of eachsheet.

Fabricated samples in accordance with the present invention were testedfor mechanical properties. The diagram in FIG. 11 shows the locationsand orientations of samples taken for the different tests.

Results from tensile testing in the L, LT and 45 directions are shown inFIG. 12. Alloy 367 listed in Table 1 showed the highest strength in allthree directions. However, the other alloys listed in Table 1 alsoexhibited favorable strength levels.

Fracture toughness tests were conducted from 16 by 44 inch center notchspecimens with 4 inch initial center cracks. FIGS. 13 and 14 illustrateR-curves from fracture toughness testing, showing that the testspecimens of the present sheet products possess favorable fracturetoughness values comparable to alclad 2524 T3 sheet. The R curves arecomparable for all of the alloys tested.

The improved strength/toughness combinations attained are shown in FIG.15. FIG. 15 also shows an average value from 2524-T3 plant fabricatedalclad sheet for comparison purposes. The minimum values shown in FIG.15 correspond to a minus 3 times the standard deviation extrapolatedvalue.

Fatigue testing under constant amplitude is shown in FIG. 16. Thesetests were conducted in samples that appeared to be most promising fromthe strength and toughness tests. These results revealed that theproducts made according to the present invention exhibit substantiallylower rates of crack growth, i.e., improved resistance to fatigue crackgrowth.

Samples in the T36 temper exhibited the properties shown in FIG. 17. InFIG. 17, the T36 temper was attained by providing 5 percent colddeformation either via cold rolling stretching. The strengths of thecold rolled samples are slightly higher.

The results from the foregoing tests revealed that the strength and theresistance to fatigue crack growth were substantially improved inaccordance with the present invention. By hot rolling at relatively hightemperatures using recovery anneals, and by adding Zr and/or Sc asdispersoid forming additions, it was possible to fabricateunrecrystallized microstructures in sheet gauges. The Li additions alsoappear to aid in the attainment of the unrecrystallized microstructuresfor unknown reasons. In 2xxx alloys, copper appears to have asubstantial effect on strengthening. Scandium additions help attainunrecrystallized microstructures but may be detrimental forstrengthening. Manganese additions are beneficial for strengthproperties. Cold rolling, e.g., 5 percent increases the strengthsignificantly without a reduction in fatigue or fracture toughness, thisalso was a surprise. Alloys containing Li may exhibit largerimprovements in properties as a result of the cold deformation thanalloys without the Li addition.

A plant rolling trial was performed with the object of producing ananisotropic grain microstructure in a sheet product to exhibit higherstrength and higher resistance to the propagation of fatigue cracks. Thealloys shown in Table 2 were cast as 15,000 lb ingots and fabricated inaccordance with the methods of the present invention, using afabrication route similar to that shown in FIG. 2. TABLE 2 Sheet ProductAlloy Compositions (Weight Percent) Alloy Sample No. Cu Mn Mg Zr Fe SiAl 354-371 4.08 0.29 1.36 0.12 0.02 0.01 balance (low Cu-low Mn) 354-3814.33 0.30 1.38 0.10 0.01 0.00 balance (high Cu-low Mn) 354-391 4.09 0.581.35 0.11 0.02 0.01 balance (low Cu-high Mn) 354-401 4.22 0.60 1.32 0.100.01 0.01 balance (high Cu-high Mn)

The sheet products having compositions listed in Table 2 were made asfollows. Ingots measuring 14 inches×74 inches×180 inches were cast usingdirect chill (DC) molds. The compositions reported in Table 2 weremeasured from metal samples obtained during casting. Ingots were firststress relieved by heating to 750° F. for 6 hours. The ingots were thenscalped to remove 0.50 inch surface layer from both rolling surfaces.For preheating, ingots were heated to 850° F., soaked for 2 hours, thenheated to 875° F. and soaked an additional 2 hours. Ingots taken fromthe preheating furnace were roll bonded to alcald 1100 plate and rolledto 6.24 inch gauge. The alcald 6.24 inch slab was reheated to 915° F.for 8 hours, table cooled to 850° F. and hot rolled to 0.180 inch gauge.Metal temperature was kept above 600° F. After hot rolling, the sheetproduct was given a recrystallization anneal at 700° F. for 8 hoursprior to solution heat treatment. The sheet product was batch solutionheat treated at 925° F. for 11 minutes and water quenched. Sheet wasflattened with a gauge reduction from 0.180 inch to 0.17746 inch. ThenT3 and T36 tempers were fabricated. The aluminum cladding had athickness of 2.5 percent of the final thickness. The anisotropicmicrostructures comprising elongated recrystallized grains attained inthe final T3 temper are shown in FIGS. 18-21.

Results from tensile strength measurements are shown in FIG. 22.Measurements of tensile properties indicated that the high Mn variantslisted in Table 2 exhibited higher strengths than the low Mn variants.The strengthening effect of Mn was surprisingly higher than that of Cu.

Fracture toughness measurements were conducted using 16 inch by 44 inchcenter notch toughness specimens. Results from strength and toughnessmeasurements are shown in FIGS. 23 to 26. These figures also show anaverage value for 2524-T3 alclad sheet for comparison purposes. Theminimum values shown in these figures correspond to a minus 3 times thestandard deviation extrapolated value. The strength and toughnesscombinations of the sheet products with high Mn variants are better thanthose of 2524-T3. Surprisingly, the low Cu-high Mn sample exhibitshigher properties than the high Cu-low Mn sample.

FIG. 27 shows the da/dN performance of the low Cu-high Mn variant forthe T3 and T36 tempers. The tests were conducted in duplicate andresulted in good correlation from the duplicate tests. Note that theseresults indicate that, at a delta K of 10, the rate of growth of fatiguecracks is reduced for the T3 tempers and reduced even more for the T36tempers. These results indicate that the products fabricated inaccordance with the present invention exhibit better FCG performance.

FIG. 28 shows results from the testing of S/N fatigue. Note that for agiven value of the number of cycles, the maximum stress is higher forproducts fabricated in accordance with the present invention. This meansthat components can be subjected to higher stresses than conventionalcomponents to experience the same life. The S/N fatigue performance ofthe products fabricated in accordance with this invention is also betterthan that of alclad 2524-T3 sheet product.

Table 3 shows the results from compressive yield strength tests, inwhich compressive strength properties in the longitudinal (L) and longtransverse (LT) orientations for alloy 2524 and one of the alloys of thepresent invention (the low Cu-high Mn variant 354-391) are compared. Asignificant improvement in compressive yield strength properties isachieved by the present sheet products in comparison with theconventional 2524 sheet product. TABLE 3 Measured Compressive YieldStrengths for Alloy 2524 and 354-391 Low Cu-High Mn Gauge L (ksi) LT(ksi) Temper 2524-T3 Measurements 0.200 42.8 49.3 T3 0.200 43.0 48.4 T30.249 42.9 48.7 T3 0.249 42.2 47.3 T3 0.249 42.5 48.5 T3 0.249 43.7 49.2T3 0.310 40.9 44.4 T3 MLHDBK5 39.0 43.0 T3 354-391 Measurements 0.17751.5 54.8 T3 0.177 51.5 56.2 T3 0.177 54.1 60.5 T36 0.177 55.2 62.1 T36

The anisotropic microstructures of some recrystallized andunrecrystallized sheet products of the present invention were measuredin comparison with conventional alloy 2024 and 2524 sheet products.Table 4 lists the Brass and Goss texture components of 2024-T3 and2524-T4 sheet products in 0.0125 inch gauges. These are compared withthe 770-309 and 770-311 unrecrystallized sheet products of the presentinvention listed in Table 1, and the 354-391 and 354-401 recrystallizedsheet products of the present invention listed in Table 2. TABLE 4Maximum Intensity of Texture Components (X Times Random) AlloyMicrostructure Brass Goss 2024-T3 recrystallized equiaxed grains 1.012.0 2524-T4 recrystallized equiaxed grains 1.9 15.3 770-309unrecrystallized elongated grains 36.1 0 770-311 unrecrystallizedelongated grains 34.9 0 354-391 recrystallized elongated grains 1.3 42.7354-401 recrystallized elongated grains 8.6 56.7

As shown in Table 4, the unrecrystallized sheet samples 770-309 and770-311 of the present invention possess Brass texture components ofgreater than 30, indicating their highly anisotropic microstructures.The recrystallized sheet samples 354-391 and 354-401 of the presentinvention possess Goss texture components of greater than 40, well abovethe Goss texture components of the conventional 2024-T3 and 2524-T4recrystallized sheet products.

The products and methods of the present invention provide severaladvantages over conventionally fabricated aluminum products. Inaccordance with the present invention, aluminum sheet productscontaining high anisotropy in grain microstructure are provided whichexhibit high fracture surface roughness and secondary cracking andbranching, making the products better suited for applications requiringlow fatigue crack growth. In addition, the products exhibit favorablecombinations of strength and fracture toughness.

Whereas particular embodiments of this invention have been describedabove for purposes of illustration, it will be evident to those skilledin the art that numerous variations of the details of the presentinvention may be made without departing from the invention as defined inthe appended claims.

1-100. (canceled)
 101. a method of making an aluminum allov sheetproduct, the method: comprising: providing an aluminum alloy; hotrolling the aluminum alloy to form a sheet; recovery annealing the hotrolled sheet; solution heat treating the recovery annealed sheet; andrecovering a sheet product comprising an anisotropic microstructuredefined by grains having an average length to width aspect ratio ofgreater than about 4 to
 1. 102. The method of claim 101, wherein therecovery anneal is performed at a temperature of from about 300 to about1,000° F. for a time of from about 0.5 to about 96 hours.
 103. Themethod of claim 101, wherein the recovery anneal is performed at atemperature of from about 400 to about 700° F.
 104. The method of claim103, wherein the sheet product is unrecrystallized.
 105. The method ofclaim 101, wherein the recovery anneal is performed at a temperature offrom about 600 to about 1,000° F.
 106. The method of claim 105, whereinthe sheet product is recrystallized.
 107. The method of claim 101,further comprising intermediate annealing the sheet prior to therecovery anneal.
 108. The method of claim 107, wherein the intermediateanneal is performed during the hot rolling.
 109. The method of claim107, wherein the intermediate anneal is performed at a temperature offrom about 400 to about 1,000° F.
 110. The method of claim 101, furthercomprising cold rolling the sheet after the hot rolling.
 111. The methodof claim 110, further comprising intermediate annealing the, sheet priorto the recovery anneal.
 112. The method of claim 111, wherein theintermediate anneal is performed during the cold rolling.
 113. Themethod of claim 112, further comprising intermediate annealing duringthe hot rolling.
 114. The method of claim 110, wherein the intermediateanneal is performed at a temperature of from about 400 to about 1,000°F.
 115. The method of claim
 101. wherein the hot rolling step includesmultiple hot rolling operations.
 116. The method of claim 115, whereinthe hot rolling operations include finish hot rolling prior to therecovery anneal.
 117. The method of claim 116, further comprisingintermediate annealing the sheet during the finish hot rolling.
 118. Themethod of claim 101, further comprising cold working the solution heattreated sheet.
 119. The method of claim 101, wherein the aluminum alloyis an Al—Cu alloy comprising aluminum, from about 1 to about 5 weightpercent Cu, up to about 6 weight percent Mg, up to about 1 weightpercent Mn, and up to about 0.5 weight percent Zr.
 120. The method ofclaim 101, wherein the Al—Cu base alloy comprises at least about 3weight percent Cu.
 121. The method of claim 101, wherein the Al—Cu basealloy includes from about 3.5 to about 4.5 weight percent Cu, from about0.6 to about 1.6 weight percent Mg, from about 0.3 to about 0.7 weightpercent Mn, and from about 0.08 to about 0.13 weight percent Zr. 122.The method of claim 101, wherein the Al—Cu base alloy includes fromabout 3.8 to about 4.4 weight percent Cu, from about 0.3 to about 0.7weight percent Mn, from about 1.0 to about 1.6 weight percent Mg, andfrom about 0.09 to about 0.12 weight percent Zr.
 123. The method ofclaim 101, wherein the Al—Cu base alloy includes from about 3.4 to about4.0 weight percent Cu, from 0 to about 0.4 weight percent Mn, from about1.0 to about 1.6 weight percent Mg, and from about 0.09 to about 0.12weight percent Zr.
 124. The method of claim 101, wherein the Al—Cu basealloy includes from about 3.2 to about 3.8 weight percent Cu, from about0.3 to about 0.7 weight percent Mn, from about 1.0 to about 1.6 weightpercent Mg, from about 0.09 to about 0.12 weight percent Zr, and fromabout 0.25 to about 0.75 weight percent Li.
 125. The method of claim101, wherein the aluminum alloy is an Al—Mg base alloy comprisingaluminum, from about 0.2 to about 7 weight percent Mg, from 0 to about 1weight percent Mn, from 0 to about 1.5 weight percent Cu, from 0 toabout 3 weight percent Zn, and from 0 to about 0.5 weight percent Si.126. The method of claim 101, wherein the aluminum alloy is an Al—Mg—Sibase alloy comprising aluminum, from about 0.1 to about 2.5 weightpercent Mg, from about 0.1 to about 2.5 weight percent Si, from 0 toabout 2 weight percent Cu, from 0 to about 3 weight percent Zn, and from0 to about 1 weight percent Li.
 127. The method of claim 101, whereinthe aluminum alloy is an Al—Zn base alloy comprising aluminum, fromabout 1 to about 10 weight percent Zn, from about 0.1 to about 3 weightpercent Cu, from about 0.1 to about 3 weight percent Mg, from 0 to about2 weight percent Li, and from 0 to about 2 weight percent Ag.
 128. Themethod of claim 101, wherein the sheet product has a thickness of up toabout 0.35 inch.
 129. The method of claim 101, wherein the sheet productis unrecrystallized.
 130. The method of claim 129, wherein theunrecrystallized sheet product has a Brass texture of greater than 20.131. The method of claim 129, wherein the unrecrystallized sheet producthas a Brass texture of greater than
 30. 132. The method of claim 129,wherein the unrecrystallized sheet product has a Brass texture ofgreater than
 40. 133. The method of claim 101, wherein the sheet productis recrystallized.
 134. The method of claim 133, wherein therecrystallized sheet product has a Goss texture of greater than
 20. 135.The method of claim 133, wherein the recrystallized sheet product has aGoss texture of greater than
 30. 136. The method of claim 133, whereinthe recrystallized sheet product has a Goss texture of greater than 40.137. A method of making an aluminum alloy sheet product, the methodcomprising: providing an aluminum alloy; hot rolling the aluminum alloyto form a sheet; intermediate annealing the hot rolled sheet; solutionheat treating the intermediate annealed sheet; recovering a sheetproduct comprising an anisotropic microstructure defined by grainshaving an average length to width aspect ratio of greater than about 4to
 1. 138. The method of claim 137, wherein the intermediate anneal isperformed at a temperature of from about 400 to about 1,000° F.
 139. Themethod of claim 137, wherein the intermediate anneal is performed duringthe hot rolling.
 140. The method of claim 137, further comprisingrecovery annealing the sheet after the intermediate anneal and prior tothe solution heat treatment.
 141. The method of claim 137, furthercomprising cold rolling the sheet after the hot rolling.
 142. The methodof claim 141, wherein the intermediate anneal is performed during thecold rolling.
 143. The method of claim 142, further comprising recoveryannealing the sheet after the cold rolling.
 144. The method of claim142, further comprising performing another intermediate annealing duringthe hot rolling.
 145. The method of claim 144, further comprisingrecovery annealing thesheet after the cold rolling.
 146. The method ofclaim 137, further comprising cold working the solution heat treatedsheet.
 147. The method of claim 137, wherein the aluminum alloy is anAl—Cu alloy comprising aluminum, from about 1 to about 5 weight percentCu, up to about 6 weight percent Mg, up to about 1 weight percent Mn,and up to about 0.5 weight percent Zr.
 148. The method of claim 147,wherein the Al—Cu base alloy comprises at least about 3 weight percentCu.
 149. The method of claim 147, wherein the Al—Cu base alloy includesfrom about 3.5 to about 4.5 weight percent Cu, from about 0.6 to about1.6 weight percent Mg, from about 0.3 to about 0.7 weight percent Mn,and from about 0.08 to about
 0. 13 weight percent Zr.
 150. The method ofclaim 147, wherein the Al—Cu base alloy includes from about 3.8 to about4.4 weight percent Cu, from about 0.3 to about 0.7 weight percent Mn,from about 1.0 to about 1.6 weight percent Mg, and from about 0.09 toabout 0.12 weight percent Zr.
 151. The method of claim 147, wherein theAl—Cu base alloy includes from about 3.4 to about 4.0 weight percent Cu,from 0 to about 0.4 weight percent Mn, from about 1.0 to about 1.6weight percent Mg, and from about 0.09 to about 0.12 weight percent Zr.152. The method of claim 147, wherein the Al—Cu base alloy includes fromabout 3.2 to about 3.8 weight percent Cu, from about 0.3 to about 0.7weight percent Mn, from about 1.0 to about 1.6 weight percent Mg, fromabout 0.09 to about 0.12 weight percent Zr, and from about 0.25 to about0.75 weight percent Li.
 153. The method of claim 137, wherein thealuminum alloy is an Al—Mg base alloy comprising aluminum, from about0.2 to about 7 weight percent Mg, from 0 to about 1 weight percent Mn,from 0 to about 1.5 weight percent Cu, from 0 to about 3 weight percentZn, and from 0 to about 0.5 weight percent Si.
 154. The method of claim137, wherein the aluminum alloy is an Al—Mg—Si base alloy comprisingaluminum, from about 0.1 to about 2.5 weight percent Mg, from about 0.1to about 2.5 weight percent Si, from 0 to about 2 weight percent Cu,from 0 to about 3 weight percent Zn, and from 0 to about 1 weightpercent Li.
 155. The method of claim 137, wherein the aluminum alloy isan Al—Zn base alloy comprising aluminum, from about 1 to about 10 weightpercent Zn, from about 0.1 to about 3 weight percent Cu, from about 0.1to about 3 weight percent Mg, from 0 to about 2 weight percent Li, andfrom 0 to about 2 weight percent Ag.
 156. The method of claim 137,wherein the sheet product has a thickness of up to about 0.35 inch. 157.The method of claim 137, wherein the sheet product is unrecrystallized.158. The method of claim 157, wherein the unrecrystallized sheet producthas a Brass texture of greater than
 20. 159. The method of claim 157,wherein the unrecrystallized sheet product has a Brass texture ofgreater than
 30. 160. The method of claim 157, wherein theunrecrvstallized sheet product has a Brass texture of greater than 40.161. The method of claim 137, wherein the sheet product isrecrystallized.
 162. The method of claim 161, wherein the recrystallizedsheet product has a Goss texture of greater than
 20. 163. The method ofclaim 161, wherein the recrystallized sheet product has a Goss textureof greater than
 30. 164. The method of claim 161, wherein therecrystallized sheet product has a Goss texture of greater than
 40. 165.The method of claim 101, wherein the sheet product comprisesunrecrystallized grains having a Brass texture of greater than 20 and/orrecrystallized grains having a Goss texture of greater than
 20. 166. Themethod of claim 165, wherein the aluminum alloy is substantially free ofLi, and comprises a maximum of 0.7 weight percent Mn and a maximum of0.2 weight percent Si.
 167. The method of claim 165, wherein thealuminum alloy comprises from about 1 to about 5 weight percent Cu, upto about 6 weight percent Mg, up to about 0.7 weight percent Mn, up toabout 0.5 weight percent Zr, up to about 0.2 weight percent Si, and issubstantially free of Li.
 168. The method of claim 165, wherein thealuminum alloy is substantially free of Li, and comprises a maximum of4.0 weight percent Cu and a maximum of 0.2 weight percent Si.
 169. Themethod of claim 165, wherein the aluminum alloy is substantially free ofLi, and comprises a maximum of 4.0 weight percent Cu, a maximum of 0.7weight percent Mn and a maximum of 0.2 weight percent Si.
 170. Themethod of claim 137, wherein the sheet product comprisesunrecrystallized grains having a Brass texture of greater than 20 and/orrecrystallized grains having a Goss texture of greater than
 20. 171. Themethod of claim 170, wherein the aluminum alloy is substantially free ofLi, and comprises a maximum of 0.7 weight percent Mn and a maximum of0.2 weight percent Si.
 172. The method of claim 170, wherein thealuminum alloy comprises from about 1 to about 5 weight percent Cu, upto about 6 weight percent Mg, up to about 0.7 weight percent Mn, up toabout 0.5 weight percent Zr, up to about 0.2 weight percent Si, and issubstantially free of Li.
 173. The method of claim 170, wherein thealuminum alloy is substantially free of Li, and comprises a maximum of4.0 weight percent Cu and a maximum of 0.2 weight percent Si.
 174. Themethod of claim 170, wherein the aluminum alloy is substantially free ofLi, and comprises a maximum of 4.0 weight percent Cu, a maximum of 0.7weight percent Mn and a maximum of 0.2 weight percent Si.